ACF's don't care when you give it up, only A) that you can give it up, and B) that you choose to take the ACF at the prereq level. If you don't have the ability to remove the class feature ( class variant, already traded it for another, etc), then it is an illegal choice for an ACF because you have nothing to trade to get the new thing. The only "prereq" is level the ACF is selected at. Shapeshift is selectable at First as a variant if your class features include an animal companion feature and a wildshape feature, both of which are on a Wildshape variant Ranger's list of class abilities. If it was important you have the feature at the level you take the ACF, then Druid's couldn't take shapeshift because they don't get Wildshape until 5th. If the ACF had a dual tier prereq of "Animal companion at first, wildshape at 5th", then I would agree about late entry for rangers, but it doesn't.